1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery of a cylindrical configuration, and more specifically to a negative electrode structure of the same.
2. Background Art
With the advancement in recent years of electronic apparatuses for their portability and cordless operation, secondary batteries such as nickel hydrogen batteries and lithium ion batteries gain attention as driving power sources because they are smaller in size, lighter in weight and higher in energy density.
A typical lithium ion secondary battery comprises a positive electrode made of a complex oxide containing lithium, a negative electrode containing a lithium metal, a lithium alloy or a negative electrode active material capable of inserting and extracting lithium ions, and an electrolyte.
There are also some researches reported in recent years on certain elements having the property of inserting lithium ions and theoretical capacity densities exceeding 833 mAh/cm3 in place of carbon materials such as graphite that have been used conventionally as the negative electrode material. Silicon (Si), tin (Sn) and germanium (Ge) having the property of alloying well with lithium, oxides and alloys of these elements are some examples of the elements used for the negative electrode active material having the theoretical capacity density exceeding 833 mAh/cm3. Of these materials, silicon particles and silicon-containing particles such as silicon oxide particles are studied broadly as less expensive alternatives.
Various kinds of lithium ion secondary batteries are proposed currently that use lithium inserting elements as the negative electrode materials. Among them, Japanese Patent Unexamined Publication, No. 2002-83594 (hereafter referred to as “patent document 1”) discloses a lithium ion secondary battery having a thin film of lithium inserting alloy deposited on a current collector by such means as the CVD method and sputtering method to form a negative electrode.
When the film-like negative electrode material is used as the one discussed above, however, the lithium inserting element exhibits a large degree of expansion and contraction due to insertion and extraction of the lithium ions. If the negative electrode used contains an active material made of Si, for instance, the negative electrode active material changes to a substance symbolized by Li4·4Si under a state of inserting lithium ions to its maximum level. A ratio of increase in volume of the active material reaches up to 4.12 times when it turns from Si to Li4·4Si. This tends to cause separation of the current collector from the negative electrode material, wrinkling and the like of the negative electrode due to weakening of adhesion between the current collector and the negative electrode material over repeated cycles of charging and discharging. It is likely that the separation and wrinkles, if occurred, degrade the charging and discharging cycle characteristics of the secondary battery.
Japanese Patent Unexamined Publication, No. 2003-303586 (“patent document 2”) is directed to address the above problem, and it discloses an electrode for secondary battery having a thin film of columnar bodies made of an active material, which are formed discretely with void spaces provided among them on a current collector. Similarly, Japanese Patent Unexamined Publication, No. 2004-127561 (“patent document 3”) discloses a secondary battery provided with a thin film of an active material formed selectively into a predetermined pattern with void spaces on a current collector of a negative electrode. It is claimed there that these structures obviate physical contacts among the adjoining columnar bodies by virtue of the void spaces provided even if the columnar bodies expand due to insertion of the lithium ions, and they can hence prevent wrinkles and fractures from being developed in the current collectors
Moreover, Japanese Patent Unexamined Publication, No. 2005-196970 (“patent document 4”) discloses a method of forming a negative electrode, wherein a current collector is provided with convex portions and concave portions on its surface, and a film-like negative electrode material is formed thereon in a tilting orientation with respect to a plane perpendicular to the main surface of the negative electrode material. It indicates that this structure can distribute stresses produced by expansion and contraction of the negative electrode material due to charges and discharges into directions of both parallel and perpendicular to the main surface of the negative electrode material, thereby preventing wrinkles and damages from being developed. In the secondary batteries shown in the patent documents 2 and 3, however, the thin films of the lithium inserting alloy are formed into either columnar bodies or in a predetermined pattern selectively with void spaces provided among them, and in an upright form in the direction of the normal to the current collector. In addition, a positive electrode active material is positioned in a manner to confront the pillar-shaped thin film as well as the void spaces through which the current collector is exposed. For this reason, the lithium delivered from the positive electrode active material deposits on the exposed current collector while also being inserted in the confronting thin film in the beginning of electrical charge. As a result, the thin film becomes less efficient in extracting the lithium during the electrical discharge. It also allows lithium metal to deposit easily, which can be the primary cause of jeopardizing the safety and decreasing the capacity. These phenomena become more pronounced as the charging and discharging cycle is repeated.
According to the secondary battery disclosed in the patent document 4, on the other hand, the negative electrode having the lithium inserting alloy formed in the tilting orientation increases the surface area confronting the positive electrode active material and also prevents the current collector from being exposed, which can use the lithium effectively and avoid deposition of the lithium metal. In other words, the secondary battery of the patent document 4 comprises negative electrode 100 having pillar-shaped negative electrode materials 150 and 160 formed in the tilting orientation on top of convex portions 130 on current collector 110, as shown in FIG. 16A to FIG. 16C, and a cylindrical secondary battery is produced by winding an electrode group formed of laminated negative electrode 100, a separator (not shown in the figures) and a positive electrode (not shown). Negative electrode materials 160 are formed in the upright orientation along a width direction of current collector 110, which is orthogonal to the winding direction, but in the tilting orientation in the winding direction, as shown in FIG. 16B and FIG. 16C. However, current collector 110 having a width W in a fully discharged state expands to an extent of δw in the width direction due to increase in volumes of negative electrode materials 150 and 160 as a result of inserting lithium ions when being charged, which causes the adjoining negative electrode materials 150 and 160 to come in contact with and push against one another, as shown in a diagrammatic illustration of FIG. 17. It is for this reason that the width of current collector 110 as well as the other relevant dimensions need to be reduced in order to make it fit into a battery case, in consideration beforehand of the dimensional changes. This gives rise to a problem that a space inside the battery case cannot be used effectively, thereby limiting improvement of the battery capacity. If any such consideration is not given to the dimensional changes, on the contrary, current collector 110 comes into abutment against insulation plates provided on the top and the bottom of the electrode group inside the battery case due to its expansion, and results in buckling or the like deformation of current collector 110. As a result, a large stress is developed in the junctions between current collector 110 and negative electrode materials 150 and 160, thereby giving rise to another problem that negative electrode materials 150 and 160 separate from current collector 110. Or, there still is a possibility that the current collector wrinkles or deforms even if negative electrode materials 150 and 160 stay not separated. There are cases as a consequence that the cycle characteristics and reliability became impaired.